(1) Field of the Invention
The present invention relates to a method for fabricating a charge-coupled device, and more particularly to a method for forming an element isolation region in cell portions of the charge-coupled device.
(2) Description of the Related Art
Plan views of a conventional charge-coupled device are given in FIGS. 1A, 1B and 1C, and sectional views thereof taken along the lines A--A in FIGS. 1A(a), 1B(a) and 1C(a) are given in FIGS. 1A(b), 1B(b) and 1C(b), respectively, for illustrating sequential steps for fabricating such a charge-coupled device. A plan view and a potential profile diagram of the charge-coupled device thus formed are given in FIGS. 2A and 2B, respectively.
First, a p-type well 2 is formed on an n-type semiconductor substrate 1. An n-type region 3 to serve as a photoelectric conversion region and an n-type region 4 to serve as a charge transfer region are formed within a surface region of the p-type well 2 by selectively introducing n-type impurities. (see FIGS. 1A(a) and 1A(b))
Next, a p.sup.+ -type region 5 is formed selectively within the p-type well 2 by highly doping with p-type impurities, thus defining the n-type region 3 and the n-type region 4. (see FIGS. 1B(a) and 1B(b))
Then, charge transfer electrodes 7 and 8 are formed on an insulating film 6 and, thereafter, an interlayer insulating film and a metal wiring are formed, whereby a conventional charge-coupled device is completed. (see FIGS. 1C(a) and 1C(b))
In the conventional fabrication method for the charge-coupled device explained above, diffusion develops laterally in the highly doped p.sup.+ -type region 5 shown in solid lines in FIG. 2A due to the effect of the thermal process which takes place subsequent to the formation of the p.sup.+ -type region 5 to serve as an element isolation region. A diffusion region 10 as shown in dash lines is formed after the completion of this process. An adverse consequence therefrom is that, since the charge transfer region at a portion adjacent to a charge-read-out gate region 9 has a wide channel width W.sub.1 and that at the remaining portion has a narrow channel width W.sub.2, there develops a potential dip .DELTA..phi..sub.ch, as illustrated in FIG. 2B (a potential profile at a plane of cross-section taken along the line B--B in FIG. 2A), which is caused by narrow channel effects during the electron-charge transferring. This potential dip deteriorates the efficiency of the charge transfer. This is a problem to be solved by the invention, in the conventional fabricating method for the charge-coupled device.